diff --git a/pkgs/by-name/un/unclutter/package.nix b/pkgs/by-name/un/unclutter/package.nix
index 6d2193e42d7e..e97a2b1d9b0f 100644
--- a/pkgs/by-name/un/unclutter/package.nix
+++ b/pkgs/by-name/un/unclutter/package.nix
@@ -15,7 +15,10 @@ stdenv.mkDerivation rec {
 
   buildInputs = [ libX11 ];
 
-  buildFlags = [ "CC=${stdenv.cc.targetPrefix}cc" ];
+  buildFlags = [
+    "CC=${stdenv.cc.targetPrefix}cc"
+    "CFLAGS=-std=c89"
+  ];
 
   installPhase = ''
     mkdir -pv "$out/bin"